2 Bible Verses about Making God A Liar

The person who [sincerely] believes in God's Son has the evidence [of it] within his heart, [confirming God's testimony]. The person who does not believe God [i.e., what He says about Jesus], accuses Him of lying, because he has not accepted the [irrefutable] testimony of God concerning [the identity and mission of] His Son.

If we claim that we have not committed sins, we are [in effect] calling God a liar [i.e., because He says we have] and His word [i.e., the real truth about the matter] is not in our hearts.